1. Field of the Invention
The present invention refers to a method of a direct voltage arc discharge enhanced treatment of at least portions of the surface of an object. A gas or gas mixture, referred to hereinbelow as a process gas, adapted to react at least partly, is fed into an evacuated main chamber containing an object to be treated. Gaseous reaction products are drawn off from the chamber and a direct voltage arc discharge is maintained in the chamber to provide a flow of charged particles.
The invention also refers to a vacuum treatment arrangement employing a vacuum chamber, and inlets for receiving a process gas introduced at least in part to react in a main chamber within the vacuum chamber. A draw-off (i.e. exhaust) arrangement is provided for removal of gaseous reaction products. An opening is provided for introducing a flow of electrically charged particles into the main chamber, and an electrode arrangement is provided for generating a direct voltage arc discharge in the main chamber.
2. Description of the Prior Art
A method and apparatus of the kind mentioned above are disclosed in Swiss Specification CH-A-664,768. In addition, U.S. Pat. No. 4,851,254 discloses a method for a reactive plasma enhanced coating of objects, in which a direct voltage arc discharge is maintained in a vacuum chamber between two closely spaced electrodes, which arc discharge operates at a low voltage, for example, below 100 V and at relatively high arc currents in the range between 50 A and 100 A.
The object to be coated is placed outside of the cathode/anode space in an insulated manner and is arranged parallel to the arc discharge. A process gas which reacts at least partly is jetted into the plasma through a pipe conduit located opposite the object to be treated with respect to the anode/cathode space.
In plasma chemical methods the reaction space is substantially limited to a region surrounding the plasma zone the extent of which is the length of the free path of the gas. The latter value is inversely proportional to the value of the pressure. A lowering of the pressure in order to increase the size of the reaction space is, however, generally not possible, because the concentration of the reactant gases obviously also decreases when the pressure is lowered.
Therefore, this procedure is deficient in that only small surfaces of the object can be coated, due to the small surface area which is adjacent the short anode/cathode space, and due to the point-like size of the jets of reactive process gas entering the chamber.
A further procedure of a reactive, plasma enhanced coating (PECO) is disclosed in U.S. Pat. No. 4,859,490. The apparatus disclosed therein includes a freely accessible glowing cathode coil, a screen connected relative thereto which is maintained at a positive, anodic potential, and an electrode located opposite the screen relative to the glowing cathode and connected cathodically relative to the screen supporting the object to be treated.
Gas, which predominantly reacts in the plasma generated in the region between the glowing coil and the screen is jetted into the central part of the main chamber and against the glowing cathode coil. A drawback of this procedure is that the reactive gas must first flow through the glowing coil and thus reacts with the glowing coil. The effect thereof is that on the one hand the coating process of the object may be disturbed by the coil and on the other hand the useful operating life of the glowing coil is drastically reduced.
An example of such a condition is the deposition of diamond coatings from hydrocarbon/hydrogen mixtures. The carburization of the filaments of refractory metal for such arrangements proceeds at such a high speed that such filaments fail due to breaking after one to three coating cycles. From the viewpoint of a controlled industrial production this is unacceptable. Similar problems are encountered in other gases such as the formation of scaling or the forming of hydrides or silicides.
The drawbacks of the method and apparatus disclosed in U.S. Pat. Nos. 4,851,254 and 4,859,490 are overcome by the procedure according to Swiss Patent No. CH-A-664,768 mentioned above, in that it is known from this document to generate a low voltage direct voltage (d.c.) arc discharge between an anode and a cathode so that electrically charged particles, electrons and ions, are fed into the discharge space making it possible to maintain an arc at low arc voltages between anode and cathode electrodes, which electrodes are separated by quite a large distance from each other compared with the separation distance according to U.S. Pat. No. 4,851,254.
In contrast to U.S. Pat. No. 4,859,490, for example, the thermally emitted electrons are not generated in the main chamber but are generated outside of the main chamber and are coupled into the main chamber through an opening arrangement, and contamination of the treatment by the material of the hot coil or the reduction of the useful operating life thereof, respectively by the treatment process is also remedied.
The drawback of the procedure according to Swiss Specification CH-A-664,768 is that in the edge area of the treatment chamber a linear jetting-out of process gas with gas is produced, or that an non-homogeneous discharge or distribution of plasma occurs in the inner space of the main chamber seen over its volume.
In many coating methods this is disturbing to such an extent that it prevents practicing of such methods. Every plasma-chemical reaction necessitates a certain plasma density, i.e. a minimal space density of charge carrying particles in which the reaction proceeds.
In the arrangement according to Swiss Specification CH-A-664,768 the space density decreases rapidly in the radial direction. An increase of arc power does not always solve this problem. The heating up of the substrates in the arc plasma is supplied predominantly by convection. The radial extent thereof is unrestricted. An increase in power causes a substantially large increase in thermal loading on the substrates than of the plasma density which is needed for deposition.